1. Field of the Invention
The present invention relates to a device for measuring the blood flow in an organ, using an injected indicator.
2. The Prior Art
Devices for measuring the blood flow in an organ, particularly the cerebral blood flow (CBF), using an injected, essentially inert indicator, have been known for a long time, and some of them are in clinical use. Devices based on older methods, such as the xenon dilution technique (Obrist, W. D., Thompson, H. K., Wang, H. S., and Wilkinson, W. F. 1975, Regional cerebral blood flow estimated by 133 xenon inhalation. Stroke 6: 245-256), are often difficult to implement, in technical terms, and prove to be time-consuming to use. Recently, technologies that use near-infrared spectroscopy (NIRS) in combination with indocyaningreen (ICG) as an indicator, in order to investigate or monitor cerebral blood flow, have gained in importance.
Generally, the use of such devices requires the invasive measurement of the input function, i.e. the arterial concentration of the indicator over time. In this connection, arterial fiber optic catheters are used, for example.
Most recently, the use of non-invasive measurement methods for monitoring cerebral blood flow have also been proposed (Keller, E., Wolf, M., Martin, M., Fandino, J. and Yonekawa, Y. 2001. Estimation of cerebral oxygenation and hemodynamics in cerebral vasospasm using indocyaningreen (ICG) dye dilution and near infrared spectroscopy (NIRS). A case report. J. Neurosurg. Anesthesiol. 13: 43-48). In this connection, optodes are affixed to the head, which function as transmitters and receivers of the near infrared radiation, by means of which the indicator concentration in the cerebral vascular system is determined. The algorithms used for evaluation are described in the literature (Keller, E., Nadler, A., Imhot, H.-G., Niederer, P., Roth, P. and Yonekawa, Y. 2002. New Methods of Monitoring Cerebral Oxygenation and Hemodynamics in Patients with Subarachnoid Hemorrhage. Acta Neurochir. [Suppl.] 82: 87-92).
A significant problem of conventional NIRS devices is that their evaluation units work with a certain inaccuracy, which arises because in the evaluation, until now, the point of departure has been the simplifying assumption that no flow of the indicator out of the system being investigated takes place during the first rise in the input signals (the so-called “rise time”). The accuracy of the results delivered within the scope of the evaluation therefore decisively depends on the sharpness of the first peak of the measurement signal after administration of the indicator. Since injection of the indicator cannot be performed in a time that is as short as desired, and at a location that is as close to the measurement location as desired, for obvious practical reasons, the dependence of the quality of the measurement results on the sharpness of the first peak of the measurement signal after administration of the indicator was accepted as being fundamental, until now.
Alternative measurement methods, such as positron emission tomography (PET), single-photo photon emission computer tomography (SPECT), or perfusion-weighted magnetic resonance spectroscopy can be implemented technically only by means of extremely expensive devices, and require transport of the patient in question to the measurement unit, in order to be used, and this transport often poses significant risk to the patient; in other words, the devices mentioned cannot be used as so-called “bedside” devices at the patient's bed.